1. Field of the Invention
The present invention relates to a load-responsive hydraulic control apparatus wherein, if hydraulic braking pressure is increased above a predetermined cut point hydraulic pressure during braking, hydraulic pressure in the rear wheel cylinders becomes less than that in the front wheel cylinders to prevent the vehicle from spinning, and wherein the cut point hydraulic pressure will vary in accordance with the vehicle load (load supported by the vehicle).
2. Related Background Art
With a known hydraulic control apparatus, if hydraulic braking pressure generated during a braking action is increased above a predetermined cut point hydraulic pressure, the hydraulic pressure in the rear wheel cylinders becomes lower than the hydraulic pressure in a master cylinder and accordingly the hydraulic pressure in the front wheel cylinders connected directly to the master cylinder (cut control) to prevent the vehicle from spinning.
When such a hydraulic control apparatus is incorporated into a so-called front and rear piping type of brake system having a front wheel brake system connecting a master cylinder and front wheel cylinders, and a rear wheel brake system connecting the master cylinder and rear wheel cylinders, the hydraulic control apparatus is provided in-line with the rear wheel brake system.
By the way, when the above-mentioned front and rear piping type of brake system is used, if the cut control of the rear wheel brake system is effected upon a malfunction of the front wheel brake system, there is a danger of lowering the braking force. To avoid this, the conventional hydraulic control apparatus is so designed that the hydraulic pressure of the front wheel brake system is introduced into the hydraulic control apparatus so that, if the hydraulic pressure of the front wheel brake system becomes substantially zero, the cut control cannot be effected (that is to say, the transmission of the hydraulic pressure between the master cylinder and the rear wheel cylinders is not prevented).
On the other hand, among the hydraulic control apparatuses, a load-responsive hydraulic control apparatus is known wherein the cut point hydraulic pressure is changed in accordance with the vehicle load (i.e., when the load is great the cut point hydraulic pressure is a higher value than when the load is small). In such a load-responsive hydraulic control apparatus, for example, since a force must be introduced into the hydraulic control apparatus in accordance with a distance between an axle of the rear wheels and the vehicle body to change the operation of the apparatus, the hydraulic control apparatus is mounted proximate the axle of the rear wheels.
However, when the hydraulic control apparatus is applied to the front and rear piping type of brake system, a pipe branched from the front wheel brake system must be connected to the hydraulic control apparatus for the purpose of detecting a malfunction of the front wheel brake system. This makes the entire system expensive. Further, since the malfunction of the front wheel brake system is detected by the hydraulic pressure in the front wheel brake system, if the hydraulic pressure in the front wheel brake system does not become zero, but the braking ability becomes impaired due for example, to fade, vapor-lock or the like, the cut control is performed, thereby further impairing the braking ability.